23
Manual Supplement
00809-0200-4853, Rev AB
Rosemount 3051S Series Pressure Transmitter
November 2017
Rosemount 3051S Series Pressure Transmitter
However, there is a combination of factors that may affect the output of the DP transmitter under single
plugged line conditions. If the impulse line is filled with an incompressible fluid, no air is present in the
impulse line or the transmitter body, and the plug is formed by rigid material, the noise or fluctuation will
decrease. This is because the combination of the above effectively “stiffens” the hydraulic system formed
by the DP sensor and the plugged impulse line. The PIL diagnostic can detect these changes in the noise
levels through the operation described previously.
6.17 Plugged line detection factors
The factors that may play a significant role in a successful or unsuccessful detection of a plugged impulse
line can be separated into positive factors and negative factors, with the former increasing the chances
of success and the latter decreasing the chances of success. Within each list, some factors are more
important than others as indicated by the relative position on the list. If an application has some negative
factors that does not mean that it is not a good candidate for the diagnostic. The diagnostic may require
more time and effort to set up and test and the chances of success may be reduced. Each factor pair will
be discussed.
Ability to test installed transmitter
The single most important positive factor is the ability to test the diagnostic after the transmitter is
installed, and while the process is operating. Virtually all DP flow and most pressure measurement
installations include a root or manifold valve for maintenance purposes. By closing the valve, preferabley
the one(s) closest to the process to most accurately replicate a plug, the user can note the response of
the diagnostic and the change in the standard deviation value and adjust the sensitivity or operation
accordingly.
Stable, in-control process
A process that is not stable or in no or poor control may be a poor candidate for the PIL diagnostic. The
diagnostic baselines the process under conditions considered to be normal. If the process is unstable, the
diagnostic will be unable to develop a representative baseline value. The diagnostic may remain in the
learning/verifying mode. If the process is stable long enough to establish a baseline, an unstable process
may result in frequent relearning/verifications and/or false trips of the diagnostic.
Well vented installation
This is an issue for liquid applications. Testing indicates that even small amounts of air trapped in the
impulse line of the pressure transmitter can have a significant effect on the operation of the diagnostic.
The small amount of air can dampen the pressure noise signal as received by the transmitter. This is
particularly true for DP devices in single line plugging situations and GP/AP devices in high pressure/low
noise applications. See the next paragraph and “Impulse line length” on page 24 for further explanation.
Liquid DP flow applications require elimination of all the air to insure the most accurate measurement.
DP Flow and low GP/AP versus high GP/AP measurements
This is best described as a (noise to signal) ratio issue and is primarily an issue for detection of plugged
lines for high GP/AP measurements. Regardless of the line pressure, flow generated noise tends to be
about the same level. This is particularly true for liquid flows. If the line pressure is high and the flow
noise is very low by comparison, there may not be enough noise in the measurement to detect the
decrease brought on by a plugged impulse line. The low noise condition is further enhanced by the
presence of air in the impulse lines and transmitter if a liquid application. The PIL diagnostic will alert the
user to this condition during the learning mode by indicating “Insufficient Dynamics” status.
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